Added context menu -- single right click on a data item in the upper
TreeView window and a context menu opens on the right, choose where to plot
the item.  This becomes the default plotting location if you later double
click on an item.

Items chosen to plot from upper TreeView window will only appear once in the
lower ListView window (checks name, title, source, and type).

Also, more code clean-up (the "this" pointer was being deleted on exit even
though "this" was created on the stack and not on the heap???).